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Select only visible vertices

Hi,

Currently I'm using O3D framework with WebGL, love it.
I am working on a 3D annotation service, where people can highlight segment parts on the 3D objects and do tagging. However, I do stumble when it comes to selecting only the polygons that are visible on the screen.

So, how will I be able to retrieved visible vertices from z buffer or depth buffer in Webgl or O3D framework as well as determining the polygon is visible in the viewport?

Currently, I am using a plane to block the ray as a temporary solution, however it still shows its limitation when the polygons are located before the plane. Doing raytracing directly onto the object to determine front faces can kill the performance, so I'm avoiding this method.